1. Technical Field of the Invention
The invention relates to the tread patterns of treads for tires intended to equip heavy vehicles and, more particularly, to the geometry of the contact surfaces with the road of the raised elements forming the tread pattern of the treads.
2. The Related Art
In order to obtain satisfactory grip performance on wet roads, it is known to provide the tread of a tire with a tread pattern formed by a plurality of grooves delimiting raised features having the form of ribs or the form of blocks. In addition, it is also possible to provide a plurality of raised elements thus formed with at least one slit in the form of a groove of small width (called an incision), or even of zero width (called a sipe) so as to create a large number of edges for cutting the layer of water covering the wet road.
In the case of tires intended to equip a driving axle of a heavy vehicle (to which axle are transmitted driving torques and braking torques), it is known to provide the treads of the tires with tread patterns formed of a plurality of blocks delimited by grooves of generally transverse and circumferential orientation. Each block of the tread pattern has a so-called leading face and a trailing face, each of which faces intersects the contact face of the block to form an edge (leading edge and trailing edge, respectively). The leading edge corresponds to a line of points of the contact face coming first as the tire contacts the road.
In view of the existence of sliding movements of low amplitude between the tread of a tire and the road when a vehicle equipped with such a tire is running, progressive and regular wear of the tread-pattern elements occurs. It has been found that this so-called “regular,” wear may be superimposed with wear which is said to be “irregular” since it is more pronounced in certain regions of the raised elements forming the tread pattern of the tread and/or on some of these elements.
This irregular wear is the result of a combination of the running mechanisms and the driving and braking forces applied intermittently to the tires (corresponding respectively to the acceleration and braking phases of the vehicle, running on roads with a slope of greater or lesser steepness). In this case, more pronounced wear of the blocks develops in the vicinity of the trailing and/or leading edges as compared to the wear found on the rest of the surface of the block (irregular wear known as “sawtooth” wear). The aforementioned running conditions lead to irregularities in the distribution of the forces exerted by the road on the contact surface of each element between the leading edge and the trailing edge.
It has additionally been found that arrangements which make it possible to improve the average rate of wear of a tire lead to irregular wear which is more marked in particular on the tread-pattern elements situated at the shoulders of the tire (that is to say close to the lateral edges of the tread). Conversely, the arrangements which make it possible to avoid irregular wear lead to an increase in the average rate of wear.
For example, the applicant's U.S. Pat. No. 5,896,905 discloses, in order to reduce the irregular wear on the tread-pattern elements of a tire tread for heavy vehicles, the formation of a heightening of the trailing edges relative to the leading edges, in combination with an inclination of the incisions. Although this results in a reduction of the irregular wear, there is an increase of the overall average wear.